This invention relates, in general, to radar systems and, more specifically, to the reduction of false point clutter alarms in ground-based radar systems.
Radar doppler filters are used in radar signal processors to attenuate echoes from undesired terrain and weather clutter. Such filters have an ability to attenuate a large portion of the clutter signals received by the radar system. However, even though a large percentage of terrain clutter is attenuated below the noise level, a significant amount of clutter residue appears at the output of the doppler filters. It is this residue which can be confusing to the signal processor of the radar system and cause it to incorrectly signal an alarm or otherwise incorrectly indicate the presence of an object in the view of the radar antenna.
In order to prevent false alarms from clutter residue appearing in doppler filter outputs, many radar systems establish a threshold value of the residue amplitude. The threshold value is the minimum value needed for an alarm to be indicated. The threshold level is established, in many cases, by the amount of clutter signal received by the zero doppler filters of the radar. According to this prior art technique, the clutter residue of the non-zero doppler filters must exceed a threshold value, derived from a fixed ratio of the clutter input to the filter bank, before an alarm is indicated.
The difficulty with this type of threshold determination is the fact that some types of clutter signals do not produce a doppler filter residue directly proportional to the applied clutter signal. In such cases, the threshold level must be adjusted to prevent alarms during the most unfavorable conditions. When this is accomplished, and since the more favorable conditions are governed by the same threshold level, the radar system's sensitivity to indicate a target is reduced.
The lack of relationship between the clutter input and the doppler filter output residue is especially prevalent when the clutter is derived from a point clutter source, such as a building or a tower. With such man-made objects, the input clutter to the doppler filters has a time-amplitude relationship similar to the antenna pattern of the radar system. As the antenna scans past the point clutter source, the skirts of the antenna radiation pattern begin to pick-up the clutter signals. The clutter signals reach a maximum when the antenna is pointing directly toward the point source. The output of the doppler filters is totally predictable, being a function of the antenna pattern, the scan rate, and the interpulse period of the radar system.
In order to use the fixed attenuation technique successfully, the false alarms on the skirts of the antenna beam were usually accepted to a certain degree, which could cause overloading of subsequent data handling stages in the radar signal processor. In general, an unsatisfactory compromise was made with most prior art systems between subclutter visibility and false alarms. The present invention drastically reduces the sacrifice in subclutter visibility necessary to reduce the number of clutter alarms to a tolerable level.
Along with point clutter, radar systems usually have apparatus for controlling the filter alarm rate due to distributed clutter residue at the output of each doppler filter, usually due to rain or extended ground clutter. Therefore, it is desirable, and it is an object of this invention, to provide a system by which the threshold level of radar doppler filters can be more responsive to the clutter residue of the filters, together with being responsive to conventional constant false alarm rate threshold determining systems.
Other prior art radar signal processors have made use of clutter maps to control the threshold when the radar antenna is synchronized with the map locations. While such techniques have been advantageous in reducing unwanted radar alarms, clutter map data is not completely responsive to the immediate state of the input clutter signal. Point clutter can vary dramatically from scan to scan; therefore the use of only clutter map data to determine the threshold for point clutter signals involves certain limitations in system performance. Therefore, it is also desirable and another object of this invention to provide a system for determining a alarm threshold level for doppler filters which is responsive to instantaneous changes in the clutter input.
Variations in the clutter residue are also dependent upon various other system parameters, such as changes in the scan rate of the antenna due to such things as wind loading, icing conditions, and motor voltage and frequency. In addition, clutter residue is dependent upon instability factors in the radar system. Therefore, it is desirable and it is another object of this invention to permit the establishment of a threshold level which takes into consideration the scan rate and the instability of the radar system.